Effect of Hygrothermal Treatment on Reliability of Thermo-Compression Bonded FPCB/RPCB Contact Joints

In this study, electrodes on a exible printed circuit board (FPCB) and rigid printed circuit board (RPCB) were bonded together by thermo-compression (TC) bonding, using a Sn-3.0Ag-0.5Cu solder as an interlayer. In order to investigate the hygrothermal reliability of the TC bonded FPCB/RPCB joints, a temperature-humidity (TH) test of 85°C/85% relative humidity, and a 90° peel test, were conducted. The relationships between the TH treatment, peel strength, and failure analysis result were discussed. The peel strength signi cantly decreased as TH time increased. In contrast, a signi cant variation in electrical resistance was not observed during TH testing. Thin and uniform (Ni,Cu)3Sn4 intermetallic compound (IMC) layers were formed at both FPCB/Sn-3.0Ag-0.5Cu/RPCB interfaces. After a TH test for 500 h, the thickness of the IMC layer was slightly increased. In the case of the joint without TH treatment, a fracture occurred at the polyimide of the FPCB. After the TH test, the degradation of the adhesion between the polyimide and Cu in the FPCB occurred, due to the hygrothermal treatment, resulting in a switch of failure mode, from a polyimide failure to a brittle polyimide/Cu interface failure. X-ray photoelectron spectroscopy (XPS) analyses showed that the decrease in C-O and C=O bond ratios caused a decrease in peel strength of the TC bonded FPCB-RPCB joints, after the TH test. When comparing the TH and HTS tests, the TH test signi cantly deteriorated the integrity of the TC bonded FPCB-RPCB joints. [doi:10.2320/matertrans.M2016006]